Union chiefs today said 96 per cent of delivery, sorting staff and Posties "opposed the privatisation of Royal Mail" in the first poll of workers on the Government's plans.

Some 92 per cent also "supported" the boycott of competitors' mail - opening the door to a full scale industrial dispute.

Royal Mail currently accepts letters, customers bills and parcels at its mail centres that have been "partially sorted" by other operators, and delivers them across the UK.

Unions have threatened a boycott of this so-called "access" mail as part of its campaign against the Government's privatisation plan.

Royal Mail went to the High Court today, hoping to obtain a ruling that such a strike would be illegal. Union chiefs have given any undertaking not to induce any action before a hearing begins next week.

The row threatens to derail the Government's plans for an early flotation of Royal Mail. Any industrial action would be the biggest since a national strike brought Royal Mail to its knees six years ago.

The CWU said 74 per cent of 112,000 Royal Mail staff voted on its consultative ballot.

Billy Hayes, CWU general secretary, said: "Today's ballot result shows we have resounding support for all the union's policies. This is a strong message to take forward action in each area to improve the working lives of postal workers and protect the services and jobs which customers and communities value."

He added: "No-one has yet asked postal workers what they think about privatisation. Today postal workers have spoken loud and clear with a massive 96 per cent roundly rejecting the Government's plans."

Business minister Michael Fallon said he would continue to meet unions to discuss the Government's plans, including the idea of issuing shares to the employees.

He said: "Everyone agrees that Royal Mail needs to change if we are to protect the future of the six-day-a-week one-price-goes-anywhere service."

The threat of protracted dispute with unions is already reported to have hit investors' interest in buying a stake in Royal Mail.

A sell-off will rival the “Tell Sid” privatisations of British Gas and BP in the mid-1980s Thatcher government. In size and money raised, it would eclipse the £1.3bn float of defence firm Qinetiq in 2005 and the £2bn listing of Railtrack in 1995.

Margaret Thatcher always opposed the sale of Royal Mail insisting she was “not prepared to have the Queen’s head privatised”.